It is a bit obscure, but there is something called Usenet where assorted posts and files can be searched for, then a Usenet Provider can download them. Usenet posts and files can be very old, which should further Kagi's mission of being a useful way to find things that have been forgotten.
In addition to this, Usenet Indexing is typically a paid service - which also happens to be part of Kagi's model. I think it would be possible to convert some Usenet users into Kagi customers, as they are already used to the concept of paid search.
Usenet - Wikipedia